A furious Winterton, whose job is to maintain discipline and unity among Labour MPs, relayed her concerns to Corbyn via a senior official in his office, saying that she regarded such attempts to label MPs as supporters and detractors as both divisive and “unacceptable”.

The intervention by Winterton follows the leaking last week of the list, in which Labour MPs were divided into five different categories ranging from a “core group” of loyalists to hard-line opponents who were labelled as “hostile”. Several other key shadow cabinet members, including Hilary Benn, the shadow foreign secretary, Lucy Powell, the shadow education secretary, and Maria Eagle, the shadow culture secretary, were listed in the second most disloyal category, “core group negative”.

Corbyn’s director of strategy and communications, Seamus Milne, insisted that while he had had a conversation with Winterton over the list it had merely been about how to handle the issue with MPs in the party who were concerned about its contents.

While Corbyn strongly denied any knowledge of the list in a conversation with Winterton, senior party sources said they believed it had been drawn up by the 19-strong group listed as the most loyal to Corbyn. Friends of Winterton say this group meets once a week to discuss internal party issues when parliament is sitting, and that Corbyn and his shadow chancellor, John McDonnell, are often present.

“It has come from that group,” said a party source, “not necessarily from the leader’s office, but from people loyal to Jeremy who meet with him and McDonnell on a regular basis. It has been massively destabilising because it looks like a blacklist.”

Winterton made it clear that such efforts to segregate MPs had not only undermined party unity but had left Labour members – many of whom had been wrongly labelled as disloyal – open to criticism from their constituents and Labour activists who were demanding why they were not more supportive.

In a further move to assert her authority, Winterton sent an email to all Labour MPs on Thursday telling them that Corbyn had assured her that his office was not involved and that he was “keen to ensure that his leadership is inclusive and reflective of the broad range of views represented in the party”. She said it was vital that MPs united around an effort to do well in May’s local elections, and ensure that the UK remained in the EU.

The emergence of the list was a gift to David Cameron at prime minister’s questions on Wednesday, when he was reeling from the resignation of Iain Duncan Smith as work and pensions secretary. While Labour MPs had hoped the party could pile into the Tories and expose their divisions, Cameron instead cited the list as evidence of Labour disunity.

Staring toward Winterton, Cameron said: “The chief whip is being a bit quiet. There are five categories – we’ve got ‘core support’, I think you can include me in that lot, we’ve got ‘core plus’. The chief whip is being a bit quiet because she’s in ‘hostile’.”

Some anti-Corbyn MPs are now warning that he will have to be replaced as leader if the party fails to make headway in May. Former frontbenchers, including Angela Smith, have said he will have to be ditched unless he can “show proper leadership”.

Deputy leader Tom Watson told the Observer: “With the Tories rupturing over Europe we have an opportunity to share our vision of a fairer society with voters who are taking a second look at Labour under Jeremy. It would be a tragedy if the opportunity is missed.”

Former shadow cabinet member Chris Leslie said Labour had to show in May that it was doing better than under Ed Miliband at the same stage of his leadership. Rather than lowering expectations, it should be powering forward and demonstrating that it was on course to win in 2020.

“Osborne has just delivered the worst of his eight budgets and the resignation of Iain Duncan Smith has cut through to the public. We have to make sure that we turn that into votes. We cannot afford to lose another election. That would be completely disastrous.”